1. Field of the Invention
The invention relates to a device and method for controlling the contrast in an image observation system comprising an image intensifier provided with a microchannel plate and an image converter which converts the output image of the image intensifier into a video signal or a visible image.
2. Brief Description of the Prior Art
Such systems are already known and an image intensifier intended for use in such a system and having a microchannel plate, hereinafter called an MCP for brevity, is disclosed, for example, in "Microchannel Plates" Mullard Technical publication M81-0151, Mullard, Redhill, Surrey, GB and in "Image Tubes" by Illes P. Csorba, Howard W. Sams and Co. Inc. (1985), Indianapolis, U.S.A. The converter may, for example, be a CCD (charge coupled device) with associated read-out electronics which delivers a video signal for display on a video screen and which, for example, observes the image of the image intensifier via an optical fibre plate or relay optics. Such a system, which therefore forms a television camera system having an image intensifier placed in front of the actual imaging device is in fact called a "low light level television camera" and will therefore hereinafter be called an LLLTV camera for the sake of brevity. The image converter may, however, also be, for example, an optical system (for example, a magnifying glass) or an additional image intensifier in a night viewer.
A drawback of the known systems is that the contrast control they contain is not adequate in those cases in which a dark scene is observed in which one or more bright points, so-called bright spots, are present. Existing contrast control systems are, in fact, based on the average brightness and in such a case they therefore attempt to keep the average brightness constant, as a result of which the dark scene portions, which often contain the information of interest to an observer, become still darker, and this is precisely what is undesirable. In addition, in the case of a video signal, the existing contrast control system is designed to adjust back the brightness in the event of bright peaks in the video signal, regardless of the average brightness, and this only increases the problem outlined.